Cross-correlated trellis-coded quadrature modulation (XTCQM) has been proposed as a generic scheme with specific embodiments that potentially offer superior alternatives to other highly power- and bandwidth-efficient phase-shift-keying (PSK) modulation schemes. Examples of such schemes include Gaussian minimum-shift keying (GMSK), staggered quadrature offset raised cosine (SQORC), Feher's patented quadrature PSK (FQPSK), and pulse-shaped offset quadrature PSK (OQPSK).

Previously developed trellis-coded-modulation (TCM) techniques combine (1) the bandwidth efficiency of such conventional multilevel-modulation techniques as multiple-phase-shift keying (MPSK) and quadrature amplitude modulation (QAM) with (2) the power efficiency of error-correction coding into (3) unified modulation schemes that, through suitable mappings, simultaneously exploit the desirable properties of (1) and (2). While the previously developed TCM techniques afford the bandwidth efficiency inherent in multilevel modulation, less attention was given, in the development of those techniques, to achieving high levels of spectral containment as quantified, for example, by keeping out-of-band power levels low to minimize adjacent-channel interference.

Considering only quadrature modulations, the innovative aspect of the development of XTCQM lies in a focus on the spectral occupancy of the transmitted signal, along with careful attention to a desirable constant-envelope property and to the power efficiency of the demodulation/decoding operation at the receiver. The more-generic nature of XTCQM (as compared with FQPSK and other schemes) affords considerably more flexibility for trading off between power and spectral efficiencies.

In XTCQM, a cross-correlation and a suitable waveform mapping are introduced into the in-phase (I) and quadrature (Q) baseband signals transmitted on quadrature carriers, in such a way as to provide a high level of spectral efficiency while also maintaining high power efficiency and a constant or pseudo-constant envelope. XTCQM would be implemented by use of such standard, currently available subsystems as an offset-QPSK modulator and a receiver containing such items as matched filters, and a Viterbi decoder. The transmitter portion of a conceptual XTCQM communication system has been shown, in a computational simulation, to perform as predicted by theory. At the time of reporting the information for this article, the software for computational simulation of the performance of the receiver portion of the system was in an early stage of development.

This work was done by Marvin K. Simon and Tsun-Yee Yan of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line www.nasatech.com/tsp at under the Electronics & Computers category. NPO-20532


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Cross-Correlated Trellis-Coded Quadrature Modulation

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This article first appeared in the August, 2000 issue of NASA Tech Briefs Magazine.

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